Abstract

The accuracy and the resolution of water-vapor measurements by use
of the ground-based differential absorption lidar (DIAL) system of
the Max-Planck-Institute (MPI) are determined. A theoretical
analysis, intercomparisons with radiosondes, and measurements in
high-altitude clouds allow the conclusion that, with the MPI DIAL
system, water-vapor measurements with a systematic error of <5% in
the whole troposphere can be performed. Special emphasis is laid on
the outstanding daytime and nighttime performance of the DIAL system in
the lower troposphere. With a time resolution of 1 min the
statistical error varies between 0.05 g/m3 in the near
range using 75 m and—depending on the meteorological
conditions—approximately 0.25 g/m3 at 2 km using 150-m
vertical resolution. When the eddy correlation method is applied,
this accuracy and resolution are sufficient to determine water-vapor
flux profiles in the convective boundary layer with a statistical error
of <10% in each data point to approximately 1700 m. The
results have contributed to the fact that the DIAL method has finally
won recognition as an excellent tool for tropospheric research, in
particular for boundary layer research and as a calibration standard
for radiosondes and satellites.

“Report of the first workshop of the World Climate Research Program/Global Energy and Water Cycle Experiment Water Vapour Project (GVaP),” 12–15 November 1996, World Climate Research Programme Informal Report No. 8 (World Meteorological Organization, Geneva, Switzerland, 1997).

“Report of the first workshop of the World Climate Research Program/Global Energy and Water Cycle Experiment Water Vapour Project (GVaP),” 12–15 November 1996, World Climate Research Programme Informal Report No. 8 (World Meteorological Organization, Geneva, Switzerland, 1997).

Relative error caused by a narrow-band interference
filter in the detector system calculated by use of Eqs. (4) and
(5) and the same data as for Fig. 2. Additionally it was set as
λ0 = 729.18 nm, Δνon,off = 50 GHz,
fl = 175 mm, and Rl(r) =
α0r + Rl,0 with
α0 = 0.

Water-vapor and temperature profiles measured during the
intercomparison experiment on 1 December 1994 at 19:13 UT. The DIAL
profile was calculated without Doppler correction. Indicated in the
DIAL profile are the statistical errors that are often less than the
diameter of the circles. Excellent agreement between the
water-vapor profiles in the 5-LSB region was achieved. Note the
extended dry layer between 800 and 1400 m.

Error analysis of the intercomparison on 1 December 1994
at 19:13 UT. The absolute and relative statistical errors of the
DIAL profile σWV and
σWV/ρWV are shown. These were
calculated with a resolution of 10 min and 60–480 m. Up to
3200 m the statistical error is less than 0.043
g/m3.

Intercomparison experiment performed on 29 April 1996 at
20:55 UT. The Doppler correction was applied in the DIAL profile
that has an effect only of approximately 10% at 2400 m. In
the DIAL profile the statistical errors are also indicated.

Error analysis of the intercomparison on 29 April 1996 at
20:55 UT. The absolute and relative statistical errors of the DIAL
profile σWV and σWV/ρWV are
shown. These were calculated with a resolution of 10 min and
75–300 m. Up to 2200 m the statistical error is less than
0.06 g/m3.

Second intercomparison experiment performed on 29 April
1996. The DIAL profile is a composite of the near-range measurement
at 22:23 UT and the far-range measurement at 23:28 UT. No Doppler
correction was applied. The statistical errors in the DIAL profile
are shown, but these are always lower than the diameter of the
circles.

Error analysis of the intercomparison on 29 April 1996 at
22:23 and 23:28 UT. The absolute and relative statistical errors of
the DIAL profile σWV and
σWV/ρWV are shown. These were
calculated with a resolution of 10 min and 75–300 m in the near range
and a resolution of 30 min and 300–900 m in the far range. Up to
2200 m the statistical error is less than 0.05
g/m3.

Backscatter signals and optical thickness
τWV of an altocumulus cloud measured on 11 September 1994
at 22:00 UT. The backscatter signals were averaged over 150 m
and 20 min. The locations of the grid points for signal averaging
and the range cell for the application of the DIAL equation are
indicated. The slope of τWV in the range cell is
proportional to the absolute humidity. A nonlinear decrease of
τWV is observed in the range where the on-line
backscatter signal is <5 LSB.

Backscatter signals and optical thickness
τWV of a cirrus cloud measured on 1 December 1994 at
20:00 UT. The backscatter signals were averaged over 50 min and
750 m. The locations of the grid points for signal averaging
and the range cell for the application of the DIAL equation are
indicated. Again, the nonlinear decrease of τWV in
the range of a low on-line signal is observed. The slope of
τWV in the range cell is proportional to the absolute
humidity.

Temperature profile measured in the upper troposphere on
1 December 1994. The locally launched Hamburg radiosonde was
started at 19:13 UT. From this radiosonde data could be received to
a resolution of 7000 m. These data were extrapolated to a
height of 11 km. A comparison with data from a radiosonde started
at 18:30 UT in Bergen, Germany, showed agreement to within 0.3 K with
the extrapolation in the cirrus cloud region.

Daytime water-vapor profile measured on 30 January 1995
at 14:40 UT (upper panel). A resolution of 10 min and 300–900
m was applied. In the lower panel the corresponding absolute and
relative statistical errors of the DIAL profile are shown. At
5000 m the statistical error is less than 0.02
g/m3.

a The specifications are fulfilled in the
field for three days.2,18,19b Δνon,off, difference
between on-line and off-line laser frequencies.c Measured after the beam expander.dRl,0, distance between the
optical axes of the laser and of the telescope at ground.eRt, radius of the telescope.f FOV, half-angle field of view of the telescope.g Δνf, FWHM of the
interference filter.

a The statistical errors in the range up to
2 km are mean values using a varying height resolution of 75 m in
the near range and 150 m in the far range. The errors in DIAL
measurements can be transformed to different resolutions by using Eqs.
(7) and (8). For a Raman lidar the transformation rule is
the same for a change in time and height resolution and it applies to
Eq. (7). PA is the product of the average power of
the laser transmitter multiplied with the area of the telescope used in
the detector system.b In the presence of strong gradients of the
inverse scattering ratio it can be higher if the Doppler correction is
not sufficient.c Depends on the accuracy of the Raman lidar
calibration.d Taking into account that daytime airborne
measurements would not be sufficiently accurate.

a The specifications are fulfilled in the
field for three days.2,18,19b Δνon,off, difference
between on-line and off-line laser frequencies.c Measured after the beam expander.dRl,0, distance between the
optical axes of the laser and of the telescope at ground.eRt, radius of the telescope.f FOV, half-angle field of view of the telescope.g Δνf, FWHM of the
interference filter.

a The statistical errors in the range up to
2 km are mean values using a varying height resolution of 75 m in
the near range and 150 m in the far range. The errors in DIAL
measurements can be transformed to different resolutions by using Eqs.
(7) and (8). For a Raman lidar the transformation rule is
the same for a change in time and height resolution and it applies to
Eq. (7). PA is the product of the average power of
the laser transmitter multiplied with the area of the telescope used in
the detector system.b In the presence of strong gradients of the
inverse scattering ratio it can be higher if the Doppler correction is
not sufficient.c Depends on the accuracy of the Raman lidar
calibration.d Taking into account that daytime airborne
measurements would not be sufficiently accurate.